This application claims the priority of German Patent Application Serial No. 198 60 466.1, filed Dec. 28, 1998.
The present invention relates, in general, to pneumatically operated hydraulic pumps.
German Pat. No. DE 26 26 954 C2 describes a pneumatically operated hydraulic pump which includes a cylinder housing for accommodating a main piston which reciprocates in response to admission of air under pressure and has attached thereto a hydraulic piston connected via a suction valve with a suction conduit and via a pressure valve with a pressure conduit. Thus, the hydraulic piston alternately executes, in response to the reciprocating motion of the main piston, a suction stroke by which hydraulic medium is aspirated, and a pump stroke by which the hydraulic medium is pumped under pressure. The reciprocating movement of the main piston is realized by a partially hollow piston-type slide valve and at least one pilot valve which is actuated by the main piston and effects a movement of the slide valve from one end position into the other end position. The slide valve has opposite end faces which are different in size to thereby provide different effective areas. The smaller end face is continuously acted upon by a working fluid under pressure, such as compressed air, while the greater end face is acted upon by working fluid via the pilot valve when the slide valve is moved in the other direction. Supply of working fluid into the working chambers on either side of the main piston and discharge of outgoing air is realized by providing a complicated, stepped sleeve in which the slide valve is guided and which has formed therein transverse bores and circumferential grooves and includes several sealing rings in spaced-apart disposition. Apart from the complexity of the sleeve, a further shortcoming of this conventional hydraulic pump is the substantial wear to which the sealing rings are subject during their continuous passage of the ports of the bores in the sleeve. In addition, the sleeve must be sealingly supported in the receiving bore of the hydraulic valve casing.
It is thus an object of the present invention to provide an improved pneumatically operated hydraulic pump, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved pneumatically operated hydraulic pump which is simple in structure and can easily be manufactured on a large scale and as a modular system.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by providing a cylinder housing of plastic having formed thereon a plastic valve bottom at one end and detachably connected thereto a plastic end cap at the other end thereof; a main piston subdividing the housing in two working chambers and movable between two end positions in response to admission of a working fluid, with the main piston having attached thereon a hydraulic piston, guidingly received in the end cap, for joint reciprocation with the main piston so that the hydraulic piston can carry out a suction stroke when the main piston moves to one end position, and a pressure stroke when the main piston moves to the other end position; a control mechanism for regulating a flow of working fluid alternately to the working chambers, with the control mechanism including a plastic valve casing which is mounted laterally to the valve bottom and has a chamber which contains working fluid under pressure, a control piston accommodated in the valve casing and having axial end faces which are different in size so as to define different effective areas and thereby permit reciprocation of the control piston by the working fluid, and a slide shoe guided in the chamber of the valve casing for conjoint movement with the control piston and having formed interiorly therein a deflection zone; and a valve arrangement which includes a first pilot valve disposed in a stepped bore of the valve bottom, and a second pilot valve disposed in a stepped bore of the end cap, and which is actuated by the main piston during displacement thereof into the two end positions, for regulating a flow of working fluid to the control mechanism to effect reciprocation of the control piston and the slide shoe so that the main piston moves between the end positions while working fluid is discharged via the deflection zone in the slide shoe to the atmosphere.
One aspect of the present invention is the fact that the cylinder housing with the integral valve bottom, the detachably secured end cap, the valve casing for the control piston and the slide shoe can all be made of plastic material, e.g. polyoxymethylene. Suitably, these components are manufactured as injection molded parts, thereby assuring a cost-efficient, large scale production of all sizes and types. Another aspect of the present invention is the operation of the slide shoe as a slide valve which can also be manufactured as an injection molded part of plastic material. Depending on its position in the valve casing, the slide shoe ensures a fluid communication of the working fluid into one or the other working chamber on both sides of the main piston and, as a consequence of the formed deflection zone, realizes a discharge of outgoing air from the working chambers into the atmosphere, suitably via a sound absorber.
The displacement of the slide shoe is implemented by a control piston which is accommodated in the valve casing and has opposite end faces of different sizes to thereby establish different effective areas. The smaller end face is always disposed in the chamber of the valve casing in which the slide shoe is also located and which is permanently acted upon by the working fluid. The control piston can be made from a light metal alloy and is sealingly guided in the valve casing. Unlike in conventional hydraulic pumps, the seals in accordance with the present invention do not move past ports and thus are not exposed to wear. The slide shoe is at all times constrained to move in the chamber but slides along the outer sidewall of the valve bottom. As the valve casing is flange-mounted to the side of the valve bottom, the operational position of the slide shoe is ensured. The slide shoe passes ports of a total of three channels which are positioned in sequence and extend transverse to the travel direction of the main piston. Both outer channels are directly connected to the working chambers of the cylinder housing, whereas the central channel communicates with the atmosphere, suitably via a sound absorber.
According to another feature of the present invention, the control piston has an elongate piston section which terminates in the smaller end face of the control piston and projects into the chamber of the valve casing, with the piston section having a recess which complements the length of the slide shoe and envelopes the slide shoe, thereby enhancing an interaction of the control piston with the slide shoe in a force-fitting and form-fitting manner. Suitably, the recess is made by suitably grooving the control piston.
According to another feature of the present invention, the chamber is in permanent fluid communication with the stepped bore, receiving the first pilot valve, via a branch duct. This ensures that in each position of the main piston the working fluid acts upon either the greater end face of the control piston via the first pilot valve as well as upon the channels in the valve bottom and in the valve casing, or the valve stem of the first pilot valve is pushed into the working chamber between the main piston and the valve bottom.
According to another feature of the present invention, the stepped bore in the valve bottom is connected to a space in the valve casing adjacent the greater end face of the control piston via channels in the valve bottom and in the valve casing and connected with the stepped bore, receiving the second pilot valve, in the end cap via channels in the wall of the cylinder housing and in the end cap. The greater end face is always located in a space of the valve casing, separated and sealed from the chamber. This space is acted upon by working fluid via the first pilot valve, which is disposed in the valve bottom, and relieved to the ambient atmosphere via the second pilot valve, disposed in the end cap. As the control piston and the slide shoe are not fixedly connected to one another, manufacture and assembly are further simplified.
In order to assure a discharge of air from the space, the stepped bore with the second pilot valve and the channels between the first pilot valve, space and second pilot valve, the stepped bore in the end cap is connected to the atmosphere via a transverse channel.
In a pneumatically operated hydraulic pump of the double acting type in which the main piston is connected to a second hydraulic piston in coaxial alignment with the first hydraulic piston, the incorporation of the second hydraulic piston can easily be carried out by simply providing a respective guide bore in the valve bottom. Other modifications are not necessary.
According to another concept of the present invention, the main piston may be interconnected with a bar linkage which transverses the valve bottom and is movable relative thereto and allows a manual displacement of the main piston in opposition to a force applied by a spring positioned between the main piston and the end cap. Thus, it is only necessary to incorporate the spring in the cylinder housing and to plug the connection between the chamber and the working chamber adjacent the end cap. Further modifications are not necessary. Thus, a hydraulic pump according to the present invention can be operated selectively with air or by hand.
Suitably, a hand lever is connected to the bar linkage and lockable in place during pneumatic operation of the hydraulic pump so that uncontrolled movements by the hand lever that may result in injury are prevented.